1. Field of the Invention
The present invention relates generally to semiconductor devices, and more specifically, to trench type power semiconductor devices.
2. Description of Related Art
Trench type power semiconductor devices such as power MOSFETs are well known. As is also known, one objective in designing these devices is to obtain a low on-resistance while also obtaining a high maximum blocking voltage, also referred to as breakdown voltage. For example, referring to FIG. 1A, which is a reproduction of FIG. 5 from U.S. Pat. No. 6,649,975, by B. J. Baliga, there is shown a prior art power MOSFET 100A. (Note that S. Sapp describes a similar device in U.S. Pat. No. 6,710,403). MOSFET 100A includes a semiconductor body 102 with a plurality of interleaved source and gate trenches formed therein, such as source trenches 132a/132b and gate trench 122. Semiconductor body 102 includes a highly doped drain region 104 of first conductivity type (e.g. N-type), a drift region 106 having a linear graded doping concentration of the same conductivity type, a channel region 108 (also referred to as a body region) of second conductivity type opposite to that of the first conductivity type (e.g., P-type), and a source region 110 of the first conductivity type.
Gate trench 122 extends within semiconductor body 102 to a depth below the bottom of channel region 108 and includes a conductive gate electrode 124 therein. Gate electrode 124 extends above and below the channel region and is insulated from semiconductor body 102 by gate insulation layer 126, which lines the side-wall and bottom of the gate trench.
Source trenches 132a/132b extend within drift region 106 of semiconductor body 102 to a depth below the bottom of gate trench 122. Within the source trenches are source electrodes 134a/134b that extend to a depth below the bottom of gate electrode 124. Source insulation layers 136a/136b line the side-wall and bottom of the source trenches and insulate the source electrodes from drift region 106.
A source contact 140 is formed over the top surface of semiconductor body 102 and electrically contacts source region 110 and source electrodes 134a/134b. Source contact 140 also contacts channel region 108 along the top surface of the device in the third dimension. A gate insulation cap 128 insulates gate electrode 124 from the source contact. A drain contact 142 is formed over the bottom surface of semiconductor body 102 and electrically contacts drain region 104.
When MOSFET 100A is operated in an on state, a gate voltage is applied to gate electrode 124. When this voltage reaches a threshold value, a vertical inversion-layer channel forms within channel region 108 along the side-wall of gate trench 122. This inversion-layer channel has the same conductivity as source region 110 and drift region 106. As a result, a current flows between source electrode 140 and drain electrode 142.
As described by Baliga, when MOSFET 100A is in an off, source electrodes 134a/134b help to improve the breakdown voltage of the device. In particular, as a reverse voltage is applied across the drain and source contacts, a depletion layer is formed as a result of the reversed-biased channel-drift junction. Because the source electrodes are in contact with the source contact, a voltage forms on these electrodes that causes the depletion layer to get pushed/spread away from the channel region and deeper into the drift region, thereby improving the blocking voltage of the device.
Notably, the source electrodes also improve the on-resistance of the device, allowing for a more highly doped drift region 106. In other words, the source electrodes allow a more highly doped drift region to now support a higher breakdown voltage than would have otherwise been possible.
Referring now to FIG. 1B, which is a reproduction of FIG. 3 from U.S. Pat. No. 5,673,898 by B. J. Baliga, there is shown another prior art power MOSFET 100B. Device 100B has a semiconductor body 102 similar to that described above, including a drift region 106 with a linear graded doping concentration that increases from channel region 108 towards drain region 104. Gate trenches, such as trench 150, are formed within the semiconductor body and extend to a depth within the drift region. A gate electrode 152 is disposed within the gate trench and is insulated from source region 110, channel region 108, and drift region 106 by gate insulation layer 154. As shown in FIG. 1B, the gate insulation layer 154 has non-uniform thickness between the trench side-wall and the gate electrode, the insulation layer being thicker along the portion of the side-wall adjacent to the drift region as compared to the portion adjacent to the channel region (this thinner insulation layer helping to maintain a low threshold voltage). A drain contact 142 is formed over the bottom surface of the semiconductor body along drain region 104. A source contact 140 is formed over the top surface of semiconductor body 102, electrically contacting source region 110. Source contact 140 also contacts channel region 108 in the top surface of the device in the third dimension.
As described by Baliga, MOSFET 100B also has improved breakdown voltage and on-resistance. In particular, the increased thickness of the gate insulation layer along the drift region improves the forward voltage blocking capability of the device by preventing high electric field crowding at the bottom corners of the trench. In addition, the increased doping of the drift region towards the drain region improves the on-resistance of the device while the reduced doping of the drift region towards the channel region improves the breakdown voltage of the device by inhibiting the occurrence of reach-through breakdown across the channel region. Nonetheless, device 100B may have a large gate-drain charge (Qgd) thereby causing a low high-frequency figure-of-merit (HFOM).
Referring now to FIG. 1C, which is a reproduction of FIG. 3 from U.S. Pat. No. 5,998,833 by B. J. Baliga, there is shown another prior art power MOSFET 100C, which is a variation of device 100B. Device 100C has a semiconductor body 102 similar to that described above, including a drift region 106 with a linear graded doping concentration. Trenches, such as trench 160, are formed within the semiconductor body and extend to a depth within the drift region. A gate electrode 162 is disposed within the upper portion of the trench adjacent channel region 108, thereby allowing for the formation of an inversion layer during the on state. A source electrode 164 is disposed in the lower portion of the trench adjacent drift region 106. An insulation layer 166 insulates the gate and source electrodes from each other and from the source region 110, channel region 108, and drift region 106. As shown in FIG. 1C, the insulation layer 166 has non-uniform thickness, being thicker along the trench side-wall and bottom adjacent to the drift region, thereby preventing high electric field crowding at the bottom corners of the trench, as similarly described above. A drain contact 142 is formed over the bottom surface of the semiconductor body along drain region 104. A source contact 140 is formed over the top surface of semiconductor body 102, electrically contacting source region 110. Source contact 140 also contacts source electrode 164 and channel region 108 in a third dimension (not shown).
As described by Baliga, as compared to device 100B, the inclusion of source electrode 164 improves the breakdown voltage of the device, the source electrode operating as similarly described for device 100A. In addition, because gate electrode 162 has a reduced size, the gate charge (Qg) and gate-drain charge (Qgd) are reduced, thereby improving the high-frequency figure-of-merit. However, as compared to device 100B, the on-resistance of device 100C is greater. In addition, it can be difficult to make contact between buried source electrode 164 and source contact 140.
In general, it is desirable to further improve the on-resistance and breakdown voltage, among other device characteristics, of a trench-type power semiconductor devices, like those shown in FIGS. 1A, 1B, and 1C.